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Solve x² - 9x + 8 = 0

The solutions of x² - 9x + 8 = 0 are x = 8 or x = 1.

x² +x += 0
x2−9x+8=0⇒x=8, 1x^2 - 9x + 8 = 0 \Rightarrow x = 8,\ 1
Discriminant D = 49 · x ≈ 8, 1

Change a, b or c (whole numbers, a not 0) and the exact solutions follow.

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Graph of y = x² - 9x + 8246810-15-10-5510152025x = 8x = 1vertexy = x² - 9x + 8
Graph of y = x² - 9x + 8. The curve crosses the x-axis at x = 8 and x = 1; the vertex is at (4.5, -12.25).

How to work it out

  1. 1

    Write it in standard form a x² + b x + c = 0 and read off the coefficients: a = 1, b = -9, c = 8.

    x2−9x+8=0x^2 - 9x + 8 = 0
  2. 2

    Work out the discriminant D = b² − 4ac. Its sign tells you how many real solutions there are.

    D=(−9)2−4⋅1⋅8=81−32=49D = (-9)^2 - 4\cdot1\cdot8 = 81 - 32 = 49
  3. 3

    D is positive, so there are two different real solutions.

  4. 4

    Method 1, factoring. The roots are rational, so the left side factors:

    (x−8)(x−1)=0\left(x - 8\right)\left(x - 1\right) = 0
  5. 5

    A product is zero when one of its factors is zero, so set each factor equal to 0 and solve:

    x−8=0⇒x=8x−1=0⇒x=1x - 8 = 0 \Rightarrow x = 8\qquad x - 1 = 0 \Rightarrow x = 1
  6. 6

    Method 2, The quadratic formula works for every quadratic:

    x=−b±D2a=9±492x = \frac{-b \pm \sqrt{D}}{2a} = \frac{9 \pm \sqrt{49}}{2}
  7. 7

    Simplify to get the solutions:

    x=8orx=1x = 8\quad\text{or}\quad x = 1
  8. 8

    Check by putting x = 8 back in: a·x² + b·x + c = 1·(64) + (-9)·(8) + 8 = 0 ✓

LaTeX source of the formulas on this page
\[ x^2 - 9x + 8 = 0\ \Rightarrow\ x = 8,\ 1 \]
\[ x^2 - 9x + 8 = 0 \]
\[ D = (-9)^2 - 4\cdot1\cdot8 = 81 - 32 = 49 \]
\[ \left(x - 8\right)\left(x - 1\right) = 0 \]
\[ x - 8 = 0 \Rightarrow x = 8\qquad x - 1 = 0 \Rightarrow x = 1 \]
\[ x = \frac{-b \pm \sqrt{D}}{2a} = \frac{9 \pm \sqrt{49}}{2} \]
\[ x = 8\quad\text{or}\quad x = 1 \]

The parabola y = x² - 9x + 8

xy
1.5-3.25
2.5-8.25
3.5-11.25
4.5-12.25
5.5-11.25
6.5-8.25
7.5-3.25

Questions

What are the solutions of x² - 9x + 8 = 0?

The solutions of x² - 9x + 8 = 0 are x = 8 or x = 1.

Does x² - 9x + 8 = 0 factor?

Yes: (x-8)(x-1) = 0.

What is the vertex of y = x² - 9x + 8?

(9/2, -49/4). The parabola opens upwards, so this is the minimum.

What are the sum and product of the roots?

Sum = −b/a = 9, product = c/a = 8.

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